1. Field of the Invention
The present invention relates to a magnetic disk unit which utilizes a magnetoresistance type head, and more specifically a method to compensate data reproduction errors due to thermal asperities, an automatic gain control system and an automatic gain control unit.
2. Description of the Prior Art
Though magnetic flux induction type magnetic heads were conventionally used in magnetic disk units, for example, magnetoresistance type heads (hereinafter referred to as MR heads) have been adopted as data is recorded at higher densities. For recording data at a high density in a magnetic recorder, on the other hand, it is necessary to shorten a distance between a head and a medium. When this distance is shortened, a protrusion or a matter such as dust adhering to the medium may cause contact between the head and the medium. Upon such a contact, the MR head is momentarily heated, thereby enhancing resistance of an MR film. This resistance change causes a phenomenon that a reproduction output from the head is abruptly changed as shown in FIG. 4. This phenomenon is known as a thermal asperity (hereinafter abbreviated as a TA). During occurrence of a TA, data cannot be discriminated and may be reproduced erroneously.
Since a gain loop used in an automatic gain controller and a timing loop used in a phase controller are generally extremely sensible of a transient DC phenomenon which is incidental to the thermal asperity, Japanese Patent Application Laid-Open No. 6-28785, for example, discloses a method, as means to minimize data reproduction errors during the occurrence of a TA, which uses a hold circuit to inactivate these feedback loops during a period of a TA.
A first problem is that the method disclosed by Japanese Patent Application Laid-Open No. 6-28785 may cause data reproduction errors since it forms a section where a gain of a variable gain amplifier (hereinafter abbreviated as VGA) is lower than an optimum level as shown in FIG. 5C when the VGA is held immediately after detecting occurrence of a TA.
The data reproduction errors are caused due to a fact that the gain is held with a delay time after the detection of the TA and an automatic gain controller (hereinafter abbreviated as an AGC) operates for this delay time, whereby a gain of the VGA has already been controlled to a low level before it is held. Further, it is general that an AGC has a slow response to a gain variation from a low level to a high level and requires a time which is longer than a time required vary a gain from a high level to a low level, whereby data reproduction errors are apt to be caused for this long response time.
A second problem is that the method does not control a gain hold of the VGA to an optimum level and allows data reproduction errors to be caused when a TA occurs immediately before reaching a data start location in a case where a gain of the VGA is held at a start time of an operation to read a certain set of data.
The data reproduction errors may be caused due to a fact that the TA occurs before the gain is held as shown in FIG. 5D and the gain has been lowered by the AGC to a low level at a start time of the gain hold operation, whereby the gain is held while it is at a level lower than an optimum level.